Method for generating optimized constraint systems for retimable digital designs

ABSTRACT

A method for generating timing constraint systems, where the constrained object is a digital circuit., is provided, where the constraints are generated for the use of a digital logic optimization (synthesis) tool. The synthesis tool is used to optimize the circuit, under the applied constraints, so that the circuit exhibits certain desirable timing properties, while at the same time minimizing hardware cost and various other properties. The particular class of timing constraints generated by the disclosed invention is useful when the circuit is to be retimed after optimization. Typically, the joint use of the described invention and retiming results in improvements in the overall cost/performance tradeoff curve of the design. The invention comprises a method that comprises the following steps: (1) the flip-flops of the design are replaced with buffers having a negative delay whose magnitude is approximately the desired clock cycle time of the design; and (2) cycles in the design are broken using flip-flops having an infinite or quasi-infinite clock frequency. Following optimization by the synthesis tool, the temporary changes can be reverted, and retiming performed on the circuit.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a method for generating optimized timingconstraint systems for retimable digital designs.

2. Description of the Prior Art

The following is a description of a generic logic synthesis system, withdescriptive emphasis being given to those aspects of the system that aremost relevant to the invention described herein. FIG. 1 shows the flowthrough a generic logic synthesis system 11 that features retiming, andthat produces a netlist 19 expressing an optimized design. The inputdesign 10 is expressed in a hardware description language (HDL), such asVerilog or VHDL. This text is analyzed, i.e. parsed and translated intoan initial circuit representation 12. In the next step 14, a clocksignal is declared, and a clock period is associated with the clocksignal. A clock is present in both combinational and sequentialcircuits. In a combinational circuit the clock signal is a dummy, butthe period must still be declared. In both the combinational andsequential cases, the clock period is used to constrain the timing ofthe circuit.

In the case of a combinational circuit, the paths being constrainedbegin at inputs and end at outputs. The clock period determines theallowable difference between the arrival time at the inputs and therequired time at the outputs. Where additional timing offsets areneeded, e.g. an additional delay on a particular input or output pin,these can be expressed as being with respect to the arrival/requiredtimes implicit in the clock period. Thus, for example, if one specifiesa clock period of 100 nanoseconds, the default delay allowed between aninput I and an output O is 100 nanoseconds. If there was a specificinput pin X with an arrival time 10 nanoseconds later than the others,one would express this as being 10 nanoseconds late with respect to thedeclared clock.

In the case of a sequential circuit, a constrained path may begin eitherat a circuit input pin, or at the Q pin of a flip-flop. Paths end eitherat circuit output pins, or at D pins of flip-flops. The path delayconstraint is the period of the clock that drives the flip-flops in thecase where a path begins at a flip-flop and ends at a flip-flop. Inother cases, the timing relationship between the I/O port and theflip-flop is computed in a manner known to practitioners of the art.

It is sometimes helpful to visualize this kind of timing constraintsystem implemented by collections of variously colored tokens. Eachtoken is colored with a color corresponding to a particular clock. Atoken is launched at either an input pin or at the Q of a flip-flopclocked by the clock whose color the token bears. It is propagatedthrough combinational logic, accumulating delay as it propagates, andfinally it arrives at, and is absorbed by, either an output pin or the Dpin of a flip-flop. The timing relationship (if there is one) betweenthe token and its final destination can then be determined by comparingthe colors of the token and the clock that constrains the flip-flop oroutput. Thus, for example, if a green token arrives at a D pin of ablue-clocked flip-flop, then the permissible delay accumulated on thetoken must be less than or equal to the worst-case time between thevalid edges of the green and blue clocks, if such a worst-case time isdefined. If a worst-case time is not defined, then the token is ignoredand no constraint is adduced.

For purposes of logic synthesis, it is also convenient to imagine asecond set of tokens being propagated backwards through the samecircuit, in a symmetrical manner, with delays being subtracted insteadof added, thus computing required times.

The term ‘slack’ is used to denote the difference between the arrivaland the required time at a particular net or pin of the circuit. Apositive slack characterizes a situation where the circuit satisfies theconstraint in question; a negative slack characterizes a situation inwhich the circuit does not.

Hence, the worst-case slack of a pin P of the circuit can be computed asbeing the minimum, taken across all colors and valid combinations ofcolors, of the difference between the arrival (forward traversal) andrequired (backward traversal) token delays, which are recorded at P asthe tokens pass through P. It is also normal practice just to speak ofthe ‘slack’ of a gate, where implicitly the slack is of the output pin(usually there is only one) of the gate in question, and the slack isthe worst-case slack.

The next step of the generic flow pictured in FIG. 1 is logic synthesis16. Here the circuit is optimized by restructuring its logic. Theprimary objective is usually to meet the timing constraints as expressedin the previous step, and the secondary objective is usually to minimizecircuit area, gate count, or some other cost function such as powerconsumption. The logic synthesis software uses the difference betweentoken arrival times and token required times to drive its decisionmaking process.

Consider, for example, the two circuits shown in FIGS. 2 a and 2 b. Thetwo circuits shown both implement the logical AND of five literals A, B,C, D, and E. In FIG. 2 a, the function is implemented by a singlefive-input gate 20, and in FIG. 2 b, by a degenerate tree 22 of fourtwo-input gates, 23-26. Neither of these circuits is intrinsicallybetter than the other. The one a synthesis tool ought to choose dependson the arrival times and slacks of the signals A-E. If, for example, allfive inputs arrive at the same time and have the same slack, the circuitshown in FIG. 2 a is probably better; whereas if E is a relativelylate-arriving, low-slack signal, then the circuit shown in FIG. 2 b isprobably better.

The example discussed above represents only one kind of optimizingdecision. There are many other kinds of optimizing decisions that alogic synthesis system can make. Each of these can be characterized bytwo or more alternative designs or classes of designs, and by a tradeoffbetween optimizing some properties and degrading others. For example,the path A-F is degraded in the design shown in FIG. 2 b of the simpleAND-gate example, whereas the path E-F is optimized. Furthermore, area,power, and other cost functions are also affected by these tradeoffs.

Some of these optimizing decision classes are:

-   <Sharing of high-level functional units such as adders, multipliers,    etc. (multiplexing inputs);-   <Speculation of high-level functional units (multiplexing outputs);-   <Implementation styles of high-level functional units, e.g. the    choice of carry-lookahead as opposed to ripple-carry adders;-   <The use of complex gates as opposed to collections of simple gates;-   <The choice of drive strength within a family of functionally    similar gates;-   <Input swapping; and-   <Functional decomposition.

These decisions have a profound effect on the overall performance andcost of the design.

Following logic synthesis is a retiming step 18. This is not alwayspresent because retiming is most useful in the context of a pipelineddesign. In retiming, registers are repositioned in the design in such away as to preserve overall functionality while optimizing the achievableclock frequency and register count.

Consider, for example, FIG. 3. In this circuit 30, the maximumachievable clock frequency is determined by the delay on the path A-F1.However, notice that if the flip-flop F1 32 is retimed through the gateG 34, in effect creating two flip-flops on the inputs of G, the maximumclock frequency is improved by the delay of G. If those two flip-flopsare then retimed further to the left (see the three routers 32′ in thecircuit 32′ shown in FIG. 4), the clock frequency improves still more.

At this point it is useful to make two observations about the genericsynthesis flow being described herein.

First, logic synthesis never changes the population of flip-flops in thedesign, except in cases where a flip-flop can be completely deletedbecause there is no path from the flip-flop's output to an output of thecircuit as a whole or as the result of a constant propagation process.

In this context, the population of flip-flops of a design can berepresented of as the set of Boolean functions that drive flip-flop Dpins. Thus, the number of functions in the population determines thenumber of flip-flops in the design, and the sequential (Mealy machine)behavior of the design is completely described by the union of theflip-flop population and the functions that drive output pins of thedesign.

Second, retiming only repositions flip-flops, and never changes thecircuit other than by deleting and inserting flip-flops. In other words,once logic synthesis has come up with a certain topology for thecombinational logic, retiming cannot change that topology. Also, onceHDL analysis or retiming has come up with a flip-flop population, logicsynthesis cannot change that population. Thus, it is easy to constructexamples of circuits where a poor choice of the initial flip-floppopulation can only be remedied with difficulty if at all.

Consider, for example, the circuits shown in FIGS. 5 a and 5 b. Ifretiming moves F 52 one gate to the left in both cases, in FIG. 5 a thecircuit 50 has five flip-flops, none of which could be removed by logicsynthesis. Whereas in the circuit 51 shown in FIG. 5 b, moving F onegate to the left results in only two flip-flops.

The salient implication of these properties is that a well-chosen set ofinitial timing constraints and flip-flop population leads to a betterdesign after retiming. Note however that it is seldom easy to constructa well-chosen constraint set and flip-flop population because the exactpositioning of the flip-flops, the delays of the gates, and the clockingconstraints collectively determine the slack of the gates and hence theretiming solution. While the delays of the gates cannot be known untilafter logic synthesis, which chooses the gates and the topology by whichthe gates are interconnected.

Another observation is that optimizing decisions are driven primarily bythe timing slack available at the various gates of the circuit. In otherwords, two copies of a small region of the circuit tend toward the samegeneral topology if the slacks are the same, almost independently of thesurrounding logic.

The constraint generation system should therefore have a property thatis henceforth called slack equivalence. Slack equivalence means that forany gate G, the timing slack at G is the same under the initialconstraint system and in an optimally retimed version of the circuit.Thus, if the entire circuit is slack-equivalent, logic synthesis seesthe same local constraints as if the circuit had already been retimed ina near-optimal way.

Slack equivalence is useful because it allows logic to be optimizedindependently of the distribution of registers in the design, and henceit alleviates the problem that logic synthesis is unable to change thepopulation of registers.

SUMMARY OF THE INVENTION

The invention provides a method for generating timing constraintsystems, where the constrained object is a digital circuit. Theconstraints are generated for the use of a digital logic optimization(synthesis) tool. The synthesis tool is used to optimize the circuit,under the applied constraints, so that the circuit exhibits certaindesirable timing properties, while at the same time minimizing hardwarecost and various other properties.

The particular class of timing constraints generated by the describedmethod is useful when the circuit is to be retimed after optimization.Typically, the joint use of the described method and retiming results inimprovements in the overall cost/performance tradeoff curve of thedesign.

The presently preferred embodiment of the method comprises the followingsteps:

-   -   1. The flip-flops of the design are replaced with buffers having        a negative delay whose magnitude is approximately the desired        clock cycle time of the design; and    -   2. Cycles in the design are broken using flip-flops having an        infinite or quasi-infinite clock frequency.

Following optimization by the synthesis tool, the temporary changes canbe reverted, and retiming performed on the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the flow through a generic logic synthesis system thatfeatures retiming, according to the prior art;

FIGS. 2 a and 2 b are schematic representations of an AND gate,according to the prior art;

FIG. 3 is a schematic representation of a circuit in which a maximumachievable clock frequency is determined by the delay on the path A-F1,according to the prior art;

FIG. 4 is a schematic representation of the circuit of FIG. 3, in whicha flip-flop F1 is retimed through a gate G, in effect creating twoflip-flops F1 a and F1 b on the inputs of G, according toe the priorart;

FIGS. 5 a and 5 b are schematic representations of circuits shown asexamples of equivalent logic functions which present different retimingissues, according to the prior art;

FIG. 6 is a schematic representation of a circuit to which a retimingalgorithm can be applied, according to the prior art;

FIG. 7 is a schematic representation of the circuit of FIG. 6 asmodified by adding negative delays and deleting registers in accordancewith the invention;

FIG. 8 is a schematic representation of an example circuit thatillustrates how the steps of the invention modify a general procedurewhen cycles are present, according to the prior art;

FIG. 9 is a schematic representation of the circuit of FIG. 8 asmodified by adding negative delays and deleting registers in accordancewith the invention;

FIG. 10 is a schematic representation of a circuit in which there is noway to break the cycle without breaking at least one forward path,according to the prior art; and

FIG. 11 is a schematic representation of a circuit in which a forwardpath may be broken according to the invention.

FIG. 12 is a diagram showing a computer.

FIG. 13 is a diagram showing various computer readable storage devices.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method for generating timing constraintsystems, where the constrained object is a digital circuit. Theconstraints are generated for the use of a digital logic optimization(synthesis) tool. The synthesis tool is used to optimize the circuit,under the applied constraints, so that the circuit exhibits certaindesirable timing properties, while at the same time minimizing hardwarecost and various other properties.

As mentioned above, one implementation of the invention is a method,This method includes certain operations, described in more detail below.In addition to this method, there are other embodiments. One example isa computer (FIG. 12) including a digital data storage and a digital dataprocessor programmed to perform the foregoing operations. A furtherembodiment relates to a computer readable storage device (FIG. 12)containing one or more programs executable by a computer to perform thestated operations.

The particular class of timing constraints generated by the describedmethod is useful when the circuit is to be retimed after optimization.Typically the joint use of the described method and retiming results inimprovements in the overall cost/performance tradeoff curve of thedesign.

The presently preferred embodiment of the method comprises the followingsteps:

-   -   1. The flip-flops of the design are replaced with buffers having        a negative delay whose magnitude is approximately the desired        clock cycle time of the design; and    -   2. Cycles in the design are broken using flip-flops having an        infinite or quasi-infinite clock frequency.

Following optimization by the synthesis tool, the temporary changes canbe reverted, and retiming performed on the circuit.

PREFERRED EMBODIMENT

The constraining method herein described is useful in the context of a(digital) logic synthesis system.

The method for generating timing constraints consists of the followingsteps. Each step is described in greater detail below:

-   -   1. The circuit is described using a standard HDL. The        descriptions should follow certain style and subset guidelines.        These guidelines, and the general practice of describing        circuits in HDL texts, are well known to practitioners of the        art.    -   2. A circuit is constructed from the HDL description. This        circuit consists of gates and flip-flops, with other features        (such as hierarchy) that are irrelevant for the purposes of this        invention. This process is also well-known.    -   3. The flip-flops of the circuit are replaced with negative        delay elements. These negative-time elements are implemented by        buffers having a delay of approximately −T, where T is a delay        equal to the flip-flop's clock period less a typical flip-flop        delay.    -   4. If the circuit contains cycles, the cycles are broken by        inserting flip-flops clocked by clocks all having a period of        zero. As a practical matter, it may be necessary to use a small        positive number, such as one femtosecond. However, this number        can usually be made so small that it makes no practical        difference. It is important to break the cycles only on backward        paths where possible.

The design so modified and constrained is now ready for logic synthesis.The steps 2-4 given above have transformed the design into a new designthat is independent of the original positioning of registers in the HDLbecause they have been removed.

The constraint system implicit in the clocks and registers constructedin steps 2-4 above has the desired property of slack equivalence, i.e.the optimization goals at each gate are close to what they would be ifthe registers were already optimally distributed.

The actual value of T can be set to the clock period of the flip-flopbeing replaced. However, some adjustments are useful, and add anincrement of accuracy to the overall constraint system.

-   -   1. The buffer used to replace the flip-flop is given a ‘typical’        load capacitance Ct, computed by taking an average or        weighted-average load capacitance over the inputs of all of the        gates and flip-flop D pins in the target technology library.    -   2. The value of T is best described using a capacitance/delay        curve representing a composite of the gates in the library, the        Q pins of the flip-flops in the library, and a series of        increasingly powerful buffer trees. The curve is first computed,        then it is offset by setting the delay corresponding to the        average load capacitance Ct to −T. Thus, a capacitive load        larger than Ct results in a longer delay, i.e. a potentially        positive net delay if the load is great enough. Conversely, if a        smaller load is imposed, the delay is −(T+t), where t is the        (positive) difference in delay between the typical load and the        lesser load.

In one alternative embodiment of the invention, it is possible toconstruct point-to-point timing constraints instead of negative-delayelement. This is straightforward for simple cases but becomes somewhatmore difficult when offset inputs and outputs, complex cycles, and/orfeed-forward paths are present.

In another alternative embodiment, a system of clocks and artificialflip-flops can be constructed that creates slack equivalence by thefollowing steps:

-   -   1. Assign inputs to stage 0.    -   2. Compute the length of the longest path from any input to each        output.    -   3. Assign the outputs to stages as denoted by the longest path        length.    -   4. Add skips (1T delays) where alternative paths are shorter        than the longest path.    -   5. Establish a series of clocks {C₀, C₁, C₂, . . . ) with a long        clock period and a phase delay of T between successive members        of the series.    -   6. For each output and input, construct a dummy flip-flop. Clock        the flip-flop with the clock whose subscript is the stage to        which the input or output has been assigned.

This alternative can also provide for loops, but the description of themethod becomes complex and the whole approach leads to some significantdifficulties.

Following logic optimization, the registers can be reinstalled in placeof the negative-delay elements, and all of the cycle-breaking flip-flopscan be removed, after which a retiming algorithm can be applied.Finally, after retiming, a second logic optimization pass can beperformed to fine-tune the retimed design.

The example circuit shown in FIG. 6 is used to illustrate how the methodis applied in a simple case. The example shows a circuit 60, shown as ablob of undifferentiated random logic, followed by three register banks61-63. For purposes of exposition, it is assumed that the blob isacyclic and contains no further registers. Further suppose that thetotal combinational delay of the combinational blob would be, if it wereoptimally synthesized, a little less than 4T. Now suppose that thedesigner intends to retime the circuit, using the three banks ofregisters to form a four-stage pipeline having a desired clock period T.The circuit as modified by adding negative delays and deleting registersin accordance with the invention looks like the circuit 70 shown in FIG.7.

The path delay beginning at the input and ending at the output is now4T−3T=T. Because the original clock period constraint is applied to theinput and output ports of the design in the usual way, i.e. such thatthe delay between an input and an output is T, the constraints and thepath delay are equal, so the circuit is slack-equivalent and tends to beoptimized in the correct way.

The property of slack equivalence is preserved by this technique for allacyclic circuits. This can be seen by considering any path beginning atan input and ending at an output. The original and retimed circuits eachhave the same number k of flops along this path; so it has −kT delay dueto the negative-delay insertion step.

FIG. 8 shows an example circuit 80 that explains how the steps of theinvention modify the general procedure when cycles are present. Thisexample shows a circuit with three blobs P, Q, and R, 81-82respectively, of combinational logic. There is one cycle C having a pathlength of one, and there is one input A and one output X.

After all of the registers 84-86 of the circuit have been replaced by −Tdelays, there is a combinational cycle beginning and ending at Q. Thesolution in this case is to add a new register 87 (FIG. 9), breaking theloop, possibly at the same location as the negative delay at C. To havethe loop delay add up correctly, this register must have a clock periodthat is substantially zero.

Now slack equivalence is preserved for a gate in P or R by the fact thatthey are both constrained only by the path beginning at A and ending atX. This path is of length 3 and delay 4T.

The logic in P is not constrained by any slack analysis involvingregister 87 because any timing-analysis tokens originating at A can onlyarrive at the output at X and the register C. The register at C issensitive only to tokens that originate at C, so other tokens have noeffect on timing analysis when they arrive at C. Similarly, such tokenscan only originate at C and only have an effect at C. When they arriveat X they are ignored.

Furthermore, the logic in the center part of Q is constrained both bythe delay going around the cycle and by the delay going from A to X. Thelogic synthesis software optimizes these gates in accordance with themost constraining of the slacks induced by the paths A-X and C-C.

Note that in the example of FIG. 9, it was possible to insert theflip-flop C in such a way that no forward path, i.e. no path from A to Xand containing no cycle, was broken. Finding such points is not alwayspossible. Consider, for example, the graph shown in FIG. 10. In thisexample there is no way to break the cycle without breaking at least oneforward path. In many other cases, it is possible to break the cyclewithout breaking the forward paths by using the following algorithm:

mark all forward paths, e.g. by using depth-first search while there areunbroken cycles choose a cycle C traverse C looking for an edge E suchthat E is not marked as forward if such an E is found, break E elsechoose an E such that it does the least damage when broken and break Eend

This algorithm breaks all cycles, so it is effective. It attempts tominimize the damage done when a forward path is broken; but for thissome heuristic must be used.

The best such heuristic known at this time is to break the forward edgeE that belongs to acyclic path from input to output with minimum oflogic on it. This minimizes the impact of the wrong decisions madeduring logic synthesis because of losing the property of slackequivalence on certain parts of the circuit. For example, for thereasons explained above it is less damaging to break the cycle at A 110,which is shown smaller in size than B to indicate less logic, ratherthan at B 111.

Although the invention is described herein with reference to thepreferred embodiment, one skilled in the art will readily appreciatethat other applications may be substituted for those set forth hereinwithout departing from the spirit and scope of the present invention.Accordingly, the invention should only be limited by the Claims includedbelow.

1. A method for generating timing constraints, comprising the steps of:describing a digital circuit using a hardware description language(HDL); preparing a digital circuit representation from said HDLdescription; removing flip-flops from said digital circuitrepresentation using a computer; replacing said removed flip-flops withnegative delay elements; and breaking any feedback paths in the digitalcircuit representation by inserting dummy flip-flops clocked by clockshaving a prescribed period.
 2. The method of claim 1, wherein saidnegative delay elements are implemented by buffers having a delay −T,where T is a delay equal to a flip-flop's clock period less apredetermined flip-flop delay.
 3. The method of claim 2, wherein T isset to a clock period of a flip-flop being replaced.
 4. The method ofclaim 1, where said step of breaking said feedback paths includesdetermining if said feedback paths can be broken without breaking anyfeedforward paths, where the breaking operation is conducted so as tomaintain feedforward paths except where said determining operationanswers in the negative.
 5. The method of claim 1, where said replacingstep is conduced so that predetermined optimization goals at each gateare substantially the same as they would be if registers were alreadyoptimally distributed.
 6. The method of claim 1, further comprising thesteps of: after said replacing and breaking steps, optimizing logic ofsaid digital circuit representation; after said optimizing, reinstallingregisters in place of said negative-delay elements and removing alldummy flip-flops; applying a retiming process to reposition registers tooptimize timing slack and register count while preserving the optimizedlogic, said operation of applying the retiming process producing aretimed design; and after retiming, applying further logic optimizationto the retimed design.
 7. The method of claim 1, where the prescribedperiod is one femtosecond or less.
 8. A method for generating timingconstraints, comprising the steps of: describing a digital circuit usinga hardware description language (HDL); preparing a digital circuitrepresentation from said HDL description; removing flip-flops from saiddigital circuit representation using a computer; replacing said removedflip-flops with negative delay elements; and where some of the negativedelay elements comprise buffers, said buffers having a load capacitancerepresenting an average or weighted-average load capacitance taken overinputs of all gates and flip-flop D pins in a target technology library.9. A method for generating timing constraints, comprising operations of:describing a digital circuit using a hardware description language(HDL); preparing a digital circuit representation from said HDLdescription; replacing flip-flops in said digital circuit representationwith negative delay elements using a computer; wherein saidnegative-time elements are implemented by buffers having a delay −T,where T is a delay equal to a flip-flop's clock period less apredetermined flip-flop delay; describing a value of T using acapacitance/delay curve representing a composite of gates in a targettechnology library, Q pins of flip-flops in said target technologylibrary, and a series of increasingly powerful buffer trees; whereinsaid curve is first computed, then it is offset by setting a delaycorresponding to a predetermined load capacitance to −T; whereby alarger capacitive load results in a longer delay; and whereby if anear-zero load is imposed a delay is −(T+t), where t is a (positive)difference in delay between a predetermined load and a lesser load. 10.A method for generating timing constraints, comprising the steps of: astep for describing a digital circuit using a hardware descriptionlanguage (HDL); a step for preparing a digital circuit representationfrom said HDL description; a step for removing flip-flops from saiddigital circuit representation and replacing said removed flip-flopswith negative delay elements; and a step for breaking any feedback pathsin the digital circuit representation by inserting dummy flip-flopsclocked by clocks all having a prescribed period using a computer.
 11. Acomputer readable storage device containing one or more programsexecutable by a computer to perform operations comprising: receiving adescription of a digital circuit using a hardware description language(HDL) and preparing a digital circuit representation from said HDLdescription; removing flip-flops from said digital circuitrepresentation and replacing said removed flip-flops with negative delayelements; and breaking any feedback paths in the digital circuitrepresentation by inserting dummy flip-flops clocked by clocks allhaving a prescribed period.
 12. A computer readable storage devicecontaining one or more programs executable by a computer to performoperations comprising: receiving a description of a digital circuitusing a hardware description language (HDL) and preparing a digitalcircuit representation from said HDL description; replacing flip-flopsin said digital circuit representation with negative delay elements;wherein said negative-time elements are implemented by buffers having adelay −T, where T is a delay equal to a flip-flop's clock period less apredetermined flip-flop delay; describing a value of T using acapacitance/delay curve representing a composite of gates in a targettechnology library, Q pins of flip-flops in said target technologylibrary, and a series of increasingly powerful buffer trees; whereinsaid curve is first computed, then it is offset by setting a delaycorresponding to a predetermined load capacitance to −T; whereby alarger capacitive load results in a longer delay; and whereby if anear-zero load is imposed a delay is −(T+t), where t is a (positive)difference in delay between a predetermined load and a lesser load. 13.A computer driven system for generating timing constraints, comprising:digital data storage; coupled to the digital data storage, a digitaldata processor programmed to perform operations comprising: receiving adescription of a digital circuit using a hardware description language(HDL) and preparing a digital circuit representation from said HDLdescription; removing flip-flops from said digital circuitrepresentation and replacing said removed flip-flops with negative delayelements; and breaking any feedback paths in the digital circuitrepresentation by inserting dummy flip-flops clocked by clocks allhaving a prescribed period.
 14. A computer readable storage devicecontaining one or more programs executable by a computer to performoperations comprising: receiving a description of a digital circuitusing a hardware description language (HDL) and preparing a digitalcircuit representation from said HDL description; removing flip-flopsfrom said digital circuit representation and replacing said removedflip-flops with negative delay elements; and where some of the negativedelay elements comprise buffers, said buffers having a load capacitancerepresenting an average or weighted-average load capacitance taken overinputs of all gates and flip-flop D pins in a target technology library.15. A computer driven system for generating timing constraints,comprising: digital data storage; coupled to the digital data storage, adigital data processor programmed to perform operations comprising:receiving a description of a digital circuit using a hardwaredescription language (HDL) and preparing a digital circuitrepresentation from said HDL description; removing flip-flops from saiddigital circuit representation and replacing said removed flip-flopswith negative delay elements; and where some of the negative delayelements comprise buffers, said buffers having a load capacitancerepresenting an average or weighted-average load capacitance taken overinputs of all gates and flip-flop D pins in a target technology library.